The next night, after Obama’s victory celebration in Des Moines, Obama strategist Steve Hildebrand spotted me in a crowd. “The kids showed up!” he said fiercely.

They did. But where are they now?

An army of 15 million voters under 30 swept Obama past Hillary Clinton and John McCain and to the presidency in 2008. More than 12 million helped him return in 2012. But now his presidency is on the line — and the Obama youth are abandoning him in his hour of need.

The administration had said it needed 40 percent of registrants in the health insurance exchanges to be young adults, or about 2.7 million of the expected 7 million total. Overall enrollment is also below target. But the alarming shortcoming is the number of young participants, which would make the insured population older and sicker and the program too expensive.

This week saw the release of Obama’s sit-down with comedian Zach Galifianakis, of “The Hangover” fame, to encourage the young to join the Obamacare exchanges. It was good comedy (the host, in the White House Diplomatic Reception Room, rolled up his sleeve to show Obama his “spider bites”), and according to the White House it had the desired result:
a boost in traffic to HealthCare.gov. Yet the fact that Obama sought Galifianakis’s help was an indication of how much the president’s standing has slipped among young Americans. Six years earlier, he had been a demigod among that demographic.

What went wrong? The president and his aides failed to keep his youth movement engaged. But part of the problem also is the inability of the millennial generation to remain attached to a cause. The generation that brought Obama to power is connected online but has no loyalty to institutions — including, it turns out, the Obama White House.

In 2008, “the level of innovation and engagement in the election, especially the primaries, was amazing, but then the level of engaging them during the administration was extremely disappointing,” says Peter Levine, a Tufts University professor who specializes in youth civic involvement. “He had a potential army for legislative success and implementation, but the Obama administration did not do that. At a critical moment in the first term, they did not turn to them. . . . They got rapid youth demobilization.”

Young voters, after playing a big role in the campaign, became little more than an e-mail list for the White House and Obama’s Organizing for Action group. Then came health-care reform. The millennials, very liberal overall, saw Obama’s plan as too timid; they were disillusioned by his failure to fight for the “public option” of government-run health plans.

This cost Obama the young activists he would need to rally enrollment in Obamacare. Polling by the nonpartisan Pew Research Center found that, while the generation looks more favorably on big-government solutions than do older generations, the millennials disapprove of Obamacare in the same proportion as the rest of the population.

Even if Obama had worked harder to keep his youth army engaged, it’s not entirely clear that the effort would have succeeded. As a group, the generation’s attachment is fickle.

A Pew survey of the generation, released earlier this month, found the 18-to-33 crowd less attached than older generations to organized politics and religion, less patriotic, less eager to marry and less trusting of people. Only 49 percent say the phrase “a patriotic person” describes them very well, compared to percentages ranging from 64 percent to 81 percent for older generations.

The millennials are at least as passionate as earlier generations and more entrepreneurial, but they lack ties to institutions — unions, political parties, churches — because of their online existence. “The organizational structure they’re growing up in is so weak,” Tufts’ Levine tells me. As a result, “there aren’t very many durable institutions that can capitalize on their enthusiasm. They’re being asked to do it themselves, online, and it’s a tall order.”

Asking them to pay money to join a health-care exchange, it seems, is too tall an order — even though the presidency they created depends on it.